phpBB : Critical Error

Error creating new session

DEBUG MODE

SQL Error : 1062 Duplicate entry '-1' for key 'session_user_id'

INSERT INTO phpbb_sessions (session_id, session_user_id, session_start, session_time, session_ip, session_page, session_logged_in) VALUES ('2cf1e8a70f41e980d6fc44bc3400bb33', -1, 1398413478, 1398413478, '36f30ec1', 13, 0)

Line : 158
File : /forum/includes/sessions.php